Soil in tire treads can spread disease

Clubroot can cut yields in half, slash quality due to smaller seeds

Canola that's been infected with clubroot. Farm equipment that hasn't been properly cleaned is believed to spread the disease.

It's the season for making tracks in farming as producers scurry from field to field, tending their crops with tillage, seeding and spraying equipment.

And with the cool, dampish spring we've had, it's almost inevitable that when farm equipment leaves a field, it will be carrying a certain amount of soil to the next one.

In the past, no one thought much about a few clumps of soil caught up in the tractor tire treads. But farmers are now advised to take all the time that's necessary to clean any muck off their equipment before leaving a field. Failure to do so could help spread a nasty soil-borne crop disease that has been making its way across the Prairies in the last decade and has surfaced here for the first time.

Clubroot was first noticed by farmers north of Edmonton in 2003. The plants were ripening prematurely, seemingly starved for moisture and nutrients. Upon examination of the roots, it became clear why. Clubroot was causing abnormal cell growth -- known as galls -- to form, choking off the plant's ability to feed itself. Canola is a member of the brassica species, which also includes vegetables such as cabbage, broccoli and turnips. They have been affected by clubroot before, but this was the first time it was found to infect canola.

By 2009, clubroot had spread to 17 counties in Alberta. It was identified in a few fields of Saskatchewan in 2011. It was initially believed the relatively high pH levels in Manitoba soils would keep it out of this province. But by 2013, Manitoba had confirmed the discovery of canola plants with clubroot galls in two unrelated fields.

Clubroot is a protist pathogen, a group of organisms that has characteristics of plants, fungi and animals, which makes it hard to control.

Severe infestations can cut yields by 50 per cent and reduce quality due to shrunken seeds. There is no spray or treatment farmers can use to protect their crops. Once it's in the soil, the spores can survive up to 20 years.

It's the same old story. Clubroot takes advantage of tight cropping rotations, just like weeds and other diseases do.

Canola prices and demand prompted farmers to grow it on the same fields every two years, and in some cases, back to back. The agronomically prudent separation is four years, although hardly anyone abides by that.

As for how it was spreading, farm equipment that hadn't been properly cleaned was identified as a key culprit. Surveys in Alberta found almost all new infestations occurred near the field access.

Plant breeders moved quickly to develop genetic resistance to the pathogen and partially resistant varieties became available in 2009. But while they reduce infections, they don't offer complete protection, and even that has proven short-lived.

Researchers warned this spring there is evidence clubroot is starting to overwhelm the resistant varieties after just four years; they had initially hoped the resistance would hold up for a decade or more. It's partly because farmers with high levels of infestation have been using the resistant varieties repeatedly. That's created selection pressure for resistant strains, similar to how resistance develops in weeds and antibiotics.

Agronomists are now advising farmers to only use the resistant varieties if they don't have clubroot.

So until a better technology can be found, farmers are back to basic agronomy -- lengthening their crop rotations and practising good equipment sanitation.

It's one thing to convince time-pressed farmers of the need to thoroughly clean equipment they use in the field and to keep supply vehicles parked on the road. What about commercial applicators, oil-industry equipment or Hydro crews also crossing their fields? They can only request their co-operation.

But there's a lot riding on the industry's ability to keep this disease in check. The council recently announced a 10-year plan to boost average farm yields to 52 bushels per acre. By comparison, last year's record production averaged 40 bushels per acre.

Diseases such as this one could be a major setback.

Laura Rance is editor of the Manitoba Co-operator. She can be reached at 792-4382 or by email: laura@fbcpublishing.com

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